1. Field of the Invention
The present invention relates to an isocyanurate polyisocyanate and its use as a curing agent for a two-component polyurethane composition. More particularly, the present invention is concerned with a novel isocyanurate polyisocyanate containing N,N',N"-tris(6-isocyanatohexyl)-isocyanurate in an extremely high proportion and having an extremely low-viscosity so that it is excellent not only in compatibility with a polyol resin having a low polarity and in solubility in a solvent having a low polarity such as n-hexane, methylcyclohexane, mineral turpentine or mineral spirit, but also in heat resistance, as compared to conventional polyisocyanates. The present invention is also concerned with a novel curing agent for a two-component polyurethane composition, which curing agent comprises mainly the present isocyanurate polyisocyanate, wherein the two-component polyurethane composition is represented by a coating composition, an adhesive composition, a casting composition, a potting composition, an elastomer composition, a foaming composition and the like. The present invention is also concerned with a novel two-component polyurethane coating composition comprising the present isocyanurate polyisocyanate and a polyol in a specific proportion, which composition is capable of not only providing an excellent coating performance but also minimizing the use of a highly polar solvent which, when formation of a coating film on a resin substrate is intended, is undesirable from the viewpoints of prevention of blistering and shrinking of the resultant coating film and corrosion of the resin substrate.
2. Discussion of Related art
Heretofore, various aliphatic polyisocyanates have been known. For example, there have been proposed a polyisocyanate having a urethane structure, which is derived from hexamethylene diisocyanate (hereinafter often referred to as "HMDI") (see Japanese Patent Application Publication Specification No. 45-11146), a polyisocyanate having a biuret structure (see U.S. Pat. Nos. 3,903,127 and 3,976,622), a polyisocyanate having an isocyanurate structure (see, for example, U.S. Pat. No. 4,324,879) and a urethane-modified polyisocyanate having an isocyanurate structure (see Japanese Patent Application Laid-open Specificaion No. 57-47321). However, each of these polyisocyanates has a viscosity as high as from 2,000 to several tens of thousands centipoises (cps) as measured at 25.degree. C.
On the other hand, two-component polyurethane coating compositions in which polyol resins such as acrylic polyols, polyester polyols or the like are used as main components and the above-mentioned aliphatic polyisocyanates derived from HMDI are used as curing agents are excellent in weatherability, flexibility, abrasion resistance, etc. and are used in various fields, such as repairing of automobiles, overcoating for architectural structures, etc.
These main components and curing agents are usually dissolved in a solvent before use. However, since the conventional aliphatic polyisocyanates are unsatisfactory with respect to solubility in a solvent having a low polarity such as mineral turpentine, mineral spirit or solvent naphtha, it is necessary to use a solvent having a high polarity such as ethyl acetate, butyl acetate or cellosolve acetate in combination with a solvent having a low polarity.
The use of a solvent having a high polarity in a large amount has drawbacks in that in case it is required to form a multi-layered coating film by repeatedly applying the coating composition, the solvent contained in the later-applied coatings tends to penetrate into the previously formed coating layers so that blistering or shrinking of the surface of the coating film is likely to occur, and that in case the coating composition is applied to a resin substrate, the surface of the resin substrate is likely to be corroded by the solvent contained in the coating composition. As opposed to the above-mentioned polyisocyanates having a viscosity of 2000 or more cps as measured at 25.degree. C., a polyisocyanate derived from HMDI and having an extremely low viscosity has been proposed in U.S. Pat. No. 4,614,785. However, such a low viscosity polyisocyanate has a drawback in that it has a high uretodione content so that free monomeric HMDI is likely to be formed during storage. Therefore, the polyisocyanate is not practically acceptable.
On the other hand, studies on polyols to be used as main components in two-component polyurethane compositions were made with respect to lowering of polarities thereof to improve the solubilities of the polyols in a solvent having a low polarity. However, polyols having low polarities have poor compatibility with conventional polyisocyanates as curing agents, leading to a lowering of the properties of the cured products, such as coating films.
Further, it is noted that in the field of coating compositions, the requirements for the properties of the ultimate coating films have recently become very severe. To meet with such requirements, various proposals have been made. For example, in U.S. Pat. No. 4,345,057 and Japanese Patent Application Laid-open Specification No. 58-34866, there have been proposed two-component polyurethane coating compositions in which fluorine-containing polyols are used as main components and polyisocyanates are used as curing agents. The compositions give ultimate coating films which are very excellent in weatherability. However, in general, the compatibility between the polyisocyanate component and the fluorine-containing polyol component is unsatisfactory. Therefore, the types of polyisocyanate components as well as types of fluorine-containing polyol components which can suitably be used for coating compositions are limited. For example, when the above-mentioned polyisocyanate having a biuret structure or the isocyanurate group-containing polyisocyanate of U.S. Pat. No. 4,324,879 is used with a fluorine-containing polyol, the coating films formed are likely to be white and turbid due to the poor compatibility between the polyisocyanate and the fluorine-containing polyol.
In this connection, it is noted that Japanese Patent Application Laid-open Specification No. 60-203674 has pointed out the poor compatibility between a polyisocyanate curing agent derived from HMDI and a fluorine-containing polyol component, and has proposed the use of a polyisocyanate having cyclohexyl rings as a polyisocyanate component in order to improve compatibility with the fluorine-containing polyol component. However, the use of a polyisocyanate having cyclohexyl rings prepared from isophorone diisocyanate or the like as a curing agent is disadvantageous in that the operational efficiency becomes poor because the polyisocyanate has an extremely high viscosity and that the flexibility of an ultimate coating film becomes poor. Accordingly, this two-component system is also not satisfactory for use as a coating composition.
Under these circumstances, it has been desired to develop an aliphatic polyisocyanate which is excellent not only in solubility even in a solvent having a low polarity but also in compatibility with a polyol having a low polarity. It has also been desired to develop a two-component polyurethane composition, especially a polyurethane coating composition in which such a polyisocyanate is used as a curing agent.